1. Field of the Invention
The present invention relates to an embedding resin for embedding electronic parts such as chip capacitors, chip inductors, chip resistances, etc., in the inside of a substrate and to a wiring substrate (wiring board) having electronic parts embedded in the inside of the substrate using the resin. Particularly, the invention is suitable for a multilayer wiring substrate, a package for containing (receiving) a semiconductor element, etc.
2. Description of the Related Art
Recently a multichip module (MCM) mounting many semiconductor elements on a build-up wiring substrate has been investigated. In the case of mounting electronic parts such as chip capacitors, chip inductors, chip resistances, etc., it is general to surface-mounting the electronic parts on a wiring layer for mounting formed on the surface of a wiring substrate using a solder.
However, when electronic parts are surface-mounted on the surface of a build-up wiring substrate, definite mounting area for various electronic parts is required, whereby there is, as a matter of course, a limit for the miniaturization. Also, by treating a wiring in the case of carrying out surface mounting, the occurrence of a parasitic inductance, which is undesirable for characteristics, is increased and there is a problem that the correspondence of electronic instruments to high frequency becomes difficult.
For solving these various problems, various methods or embedding electronic parts in the inside of a substrate have been investigated. For example, Japanese Patent Laid-Open No. 126978/1999 discloses a method of, after previously solder-mounting electronic parts to a wiring substrate having a transfer sheet made of a metal foil, transferring the electronic parts, but there remains a problem in the position precision, etc., at mounting. Also, Japanese Patent Laid-Open No. 124352/2000 disclosed a multilayer wiring substrate obtained by build-upping an insulating layer on electronic parts embedded in the inside of a core substrate.
In the method of embedding electronic parts in the inside of an insulating substrate such as a core substrate, etc., it is necessary that the gaps between the insulating substrate and the electronic parts are embedded with an embedding resin, and further a wiring layer formed on the insulating layer built-up thereon is electrically connected to the electrodes of the electronic parts by electroless plating, etc. In this case, for insuring the reliability of the connection, it is necessary to fill also fine gaps between the electrodes of the electronic parts with the embedding resin. For the purpose, it is necessary that the embedding resin has a low viscosity. Moreover, when the using environment is considered, it is necessary to prolong the usable time (the time of maintaining the good treating property of the embedding resin even when the curing reaction proceeds to some extent).
As a method of controlling the viscosity of the embedding resin, there are largely two methods. Practically, there are a method of controlling the addition amount of a filler and a method of using a curing agent having a slow curing rate.
In general, when the addition amount of a filler is reduced, the viscosity of the resin can be lowered. However, for preventing the occurrence of a trouble caused by the difference of the thermal expansion coefficient between materials, it is necessary to match the thermal expansion coefficient of the embedding resin with the expansion coefficient of a material, which becomes the core substrate or the build-up material to some extent. For the purpose, the addition of at least a definite amount of a filler is necessary. As described above, by only controlling the addition amount of filler, it was difficult to obtain both lowering the viscosity and obtaining the reliability.
An object of the invention is to provide an embedding resin capable of realizing both lowering of the viscosity and obtaining a high reliability by matching of thermal expansion coefficients.
Another object of the invention is to provide a wiring substrate wherein electronic parts disposed in the inside of an opening formed in an insulating substrate embedded therein using the embedding resin.
That is, the embedding resin of the invention is an embedding resin containing a thermoplastic resin, an acid anhydride curing agent, a curing accelerator, and a filler, wherein the viscosity thereof after allowing to stand for 24 hours at 25xc2x0 C.xc2x11xc2x0 C. can be maintained at not higher 85 Paxc2x7s in a shear rate of 8.4 sxe2x88x921.
Then, the invention is explained in detail.
About the embedding resin, when the using method thereof is considered, it is necessary to lower the viscosity thereof in one part liquid state of the mixture of the resin component, the acid anhydride curing agent, the curing accelerator, and the filler. In this case, when the workability such as filling property, etc., is considered, the embedding resin, which can maintain the viscosity thereof after allowing to stand for 24 hours at 25xc2x0 C.xc2x11xc2x0 C. at not higher than 85 Paxc2x7s, preferably not higher than 60 Paxc2x7s, and more preferably not higher than 45 Paxc2x7s in a shear rate of 8.4 sxe2x88x921, is preferred. More preferably, the embedding resin, which can maintain the viscosity thereof after allowing to stand for 48 hours at 25xc2x0 C.xc2x11xc2x0 C. at not higher than 85 Paxc2x7s, preferably not higher than 60 Paxc2x7s, and more preferably not higher than 45 Paxc2x7s in a shear rate of 8.4 sxe2x88x921, is preferred. By selecting the material, which can maintain the low viscosity for a long time, the increase of the viscosity during working at normal temperature can be restrained, whereby the occurrence of troubles such as inferior filling, etc., can be prevented and the yield can be improved.
The amount of the resin component (thermoplastic resin) is preferably 10 to 45 wt %, more preferably 10 to 23 wt %, based on the embedding resin.
As the curing agent, it is preferred to use the acid anhydride curing agent the viscosity of which at 25xc2x0 C.xc2x11xc2x0 C. is not higher than 170 mPaxc2x7s, preferably not higher than 100 mPaxc2x7s, and more preferably not higher than 60 mPaxc2x7s. The acid anhydride curing agent is a material contributing to lower the viscosity of the embedding resin. By using the curing agent having the viscosity as low as possible, the viscosity of the embedding resin itself can be lowered. In addition, since the acid anhydride curing agent having a viscosity of not higher than 170 mPaxc2x7s shows the behavior as Newtonian flow different from the embedding resin, the viscosity is not largely fluctuated by the change of a shear rate. Accordingly, the viscosity of the curing agent may be measured by a shear rate different from the shear rate (8.4 sxe2x88x921) at measuring the embedding resin.
Also, by using the curing agent having a very low viscosity, even when the curing reaction of the embedding resin proceed to some extent, the curing agent can be used at the low viscosity. (that is, the usable time is long). As the result thereof, the effects of improving the workability and capable of preventing the entrance of bubbles at filling the embedding resin are obtained. Also, since by using the curing agent having a low viscosity, the viscosity of the embedding resin can be lowered, it is desirable to use the curing agent having a low viscosity.
As the acid anhydride curing agent, phthalic anhydride-base curing agents are preferred. Particularly, methyltetrahydrophthalic anhydride or methylhexahydrophthallic anhydride is preferred because of the high storage stability.
The amount of the curing agent is preferably 10 to 45 wt %, more preferably 10 to 26.5 wt %, based on the embedding resin.
The amount of the curing accelerator is preferably 0.02 to 3.5 wt %, based on the embedding resin.
In the embedding resin of the invention, by appropriately controlling the content of the filler, the filling property thereof can be more effectively improved. The preferred content of the filler is from 45 to 90% by weight, more preferably from 51 to 74% by weight, based on the embedding resin. When the compounding ratio of the filler is less than 45% by weight, the difference of The thermal expansion coefficient with the core substrate and the material becoming the build-up material becomes large, which causes the generation of cracks at applying a heat cycle. Also, when the content of the filler exceeds 90% by weight, the viscosity of the embedding resin becomes high and the filling property thereof is greatly deteriorated to cause the entrance of bubbles.
It is preferred that the embedding resin of the invention contains at least one kind of an inorganic filler to the resin component. The reasons for adding an inorganic A filler is for the control of the thermal expansion coefficient and further for preventing the form of the embedding resin after the roughening treatment from being crumbled by the effect as the aggregate obtained by the inorganic filler.
There is no particular restriction on the inorganic filler but crystalline silica, fused silica, alumina, silicon nitride, etc., are preferred. The inorganic filler can effectively lower the thermal expansion coefficient of the embedding resin, thereby the improvement of the reliability to heat cycle is obtained.
As to the filler size of the inorganic filler, for the necessity that the embedding resin easily flows in even the gaps between the electrodes of electronic parts, the filler having the particle sizes of not larger than 50 xcexcm is preferably used. When the particle size of the filler exceeds 50 xcexcm, the filler is liable to be clogged in the gaps between the electrodes of the electronic parts and by inferior filling of the embedding resin, the portions having extremely different thermal expansion coefficient generate locally. The lower limit of the particle size of the filler is preferably at least 0.1 xcexcm. When the particle size of the filler is finer than 0.1 xcexcm, the fluidity of the embedding resin becomes hard to be insured. Thus, the particle size of the filler is preferably at least 0.3 xcexcm, and more preferably at least 0.5 xcexcm. For attaining a low viscosity and a high filling of the embedding resin, it is preferred to widen the particle size distribution.
For increasing the fluidity and the filling of the embedding resin, the form of the inorganic filler is preferably an almost spherical form. In particular, a silica-base inorganic filler is preferred since the spherical filler can be easily obtained.
It is preferred that, if necessary, the surface of the inorganic filler is subjected to a surface treatment with a coupling agent. As the kind of the coupling agent, silane-base, titanate-base, aluminate-base, etc., are used.
In the wiring substrate having electric parts built-in using the embedding resin of the invention, the electric parts are disposed in the opening formed in an insulating substrate and gaps in the opening is embedded with the embedding resin of the invention. The term xe2x80x9celectronic parts are embeddedxe2x80x9d in the invention means that after disposing electronic parts in an opening (a throughhole (shown, for example, in FIG. 2)) or a concave portion such as cavity (shown, for example, in FIG. 10) formed in a substrate such as a core substrate or a build-up insulating layer, the embedding resin is filled in the gaps formed between the opening and the electronic parts. Practically, a capacitor built-in type flip-chip package shown in FIG. 1 or FIG. 10 can be formed. In addition, not only the bump grid array type package but also a pin grid array type package can be formed. As the opening, a throughhole formed by punching the substrate or cavities, etc., formed by a multilayer technique can be preferably utilized. As the substrate, which is used in the invention, a so-called core substrate such as FR-4, FR-5, BT, etc., is preferably used but a core substrate formed by sandwiching a copper foil having a thickness of about 35 xcexcm in thermoplastic resin sheets such as PTFE sheets, etc., and having formed an opening maybe used. Also, a substrate formed by alternately laminating an insulating layer and a wiring layer on at least one surface of a core substrate to form a build-up layer and having formed an opening penetrating the core substrate and the build-up layer can be used. In this case, even d multilayer wiring substrate of a capacitor build-in type as shown in FIG. 11, there is a merit that the thickness of a so-called glass-epoxy composite material (insulating substrate) can be thinned to about 400 xcexcm, which is a half of 800 xcexcm of an ordinary product, to make a low back substrate.
In addition, the above-described electronic parts include passive electronic parts such as a chip capacitor, a chip inductor, a chip resistance, a filer, etc.; active electronic parts such as a transistor, a semiconductor element, FET, low-noise amplifier (LNA), etc., and other electronic parts such as a SAW filter, a Lc filter, an antenna switch module, a coupler, a diplexer, etc.
By sufficiently insuring the usable time at normal temperature and using the embedding resin having a low viscosity, the fine gaps between the electrodes of the electronic parts can be sufficiently filled with the embedding resin. Therefore, the wiring substrate of the invention can be used as an electronic parts built-in type wiring substrate having a high reliability to heat cycle.
A multilayer wiring substrate, wherein a build-up layer formed by alternately laminating an insulating layer and a wiring layer is formed at least one surface of a core substrate, and a substrate having formed an opening such that the opening penetrates the core substrate and the build-up layer is used, may be preferably produced, for example, as follows (FIG. 11 to FIG. 25).